Chemistry of Group IV Metal Ion-Containing Polyoxometalates

The chemistry of Group IV metal ion (Ti-IV, Zr-IV and Hf-IV)-containing polyoxometalates (POMs) is presented as a sharply focused microreview, introducing an aspect of our own research. The synthesis, structure, and solid/solution state behavior of the POMs, which are prepared by the reactions of various lacunary species of POMs with Ti-IV, Zr-IV and Hf-IV atoms, are described in several sections, classified with Keggin and Dawson POM families. Because of the ionic radius of Ti-IV (0.75 angstrom), close to that of W-VI (0.74 angstrom), the Ti-IV atom can fit nicely into the mono-lacunary site of the POM, but Zr-IV and Hf-IV atoms (0.85-0.86 angstrom), larger than Ti-IV atom, do not fit into the mono-lacunary site of the POM. Thus, the mono-lacunary site of the POM acts as the oxygen-donor pentadentate ligand to the Ti-IV atom, whereas it acts as the tetradentate ligand to Zr-IV and Hf-IV atoms. The Ti-IV atom in the POM takes on six-coordinate geometry, whereas the Zr-IV and Hf-IV atoms have higher coordination numbers (6, 7 and 8) due to their larger ionic radii. Consequently, most Ti-sub-stituted Keggin POMs are isolated as oligomers formed by corner-sharing Ti-O-Ti bonds, whereas Zr/Hf-containing Keggin/Dawson POMs are usually isolated as di-, tri-, tetra-Zr/Hf cluster cations sandwiched between two lacunary POMs, the cluster cations of which are formed by edge-sharing M(OH)(2)M (M = Zr, Hf) bonds. These compounds show quite different behavior under pH-dependent conditions. The pH-dependent interconversion between the dimeric and monomeric species of Ti-substituted Dawson POMs is quite an opposite tendency from those of Zr/Hf-containing Dawson POMs. The Ti-substituted Dawson POM oligomers have a tendency to undergo base hydrolysis to give monomers, whereas the Zr/Hf-containing Dawson POM oligomers have a tendency to undergo acid hydrolysis to provide monomers. In the Group IV metal ion-containing POMs, the Zr/Hf atoms function very similarly to each other, but show quite different behavior from the Ti atom.